The invention relates to treatment fluids for subterranean formations penetrated by a wellbore, and more particularly, crosslinkable polymer based fracturing fluids which incorporate metal oxides to help control crosslinking.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the recovery of hydrocarbons from subterranean formations it is common practice, particularly in low permeability formations, to fracture the hydrocarbon-bearing formation (i.e. to create a fracture or create a less resistance path for the formation fluids) to enhance oil and gas recovery. In such fracturing operations, a fracturing fluid that is capable of suspending a proppant is hydraulically injected into a wellbore that penetrates a subterranean formation. The fracturing fluid is forced against the formation strata by applying sufficient pressure to the extent that the fracturing fluid opens a fracture in the formation. This pressure is then maintained while injecting fracturing fluid at a sufficient rate to further extend the fracture in the formation. As the formation strata or rock is forced to crack and fracture, a proppant is placed in the fracture by movement of a viscous fluid containing proppant into the crack in the rock. After the pressure is reduced, the fracture closes on the proppant, thus preventing complete closure of the fracture. The resulting fracture, with proppant in place, provides improved flow of the recoverable fluid, i.e., oil, gas, or water, into the wellbore.
Natural polymers such as guar and its derivatives have been widely used as gelling agents to prepare viscous fracturing fluids for hydraulic fracturing treatments. These polymers are water soluble or “hydratable” polymers. For such treatments, the polymer is combined with an aqueous fluid so that the polymer is hydrated to give a linear gel solution and then crosslinked to give a viscous gel solution. Crosslinking may occur instantly or be delayed until the fluid reaches the desired location.
The most common fluid system used today mainly consists of naturally occurring guar as a gelling agent and boric acid, which is activated by a highly soluble base such as sodium hydroxide (NaOH). The amount of NaOH used is dependent upon the formation temperature in order that the fluid maintains appropriate pH fluid for stability. For example, for formation temperature of 100° C., it is common to add NaOH to achieve an initial fluid pH of 11 to 12.
In crosslinked polymer solutions, higher temperatures typically result in a drop of the pH. When the pH is reduced too much, gelation of the solution can be reversed so that fluid viscosity is lost. A higher pH is therefore typically used to ensure stability under high temperature conditions. At a pH of 10-12, which is often used for such fluids, the crosslinked polymer system typically exhibits increased stability under high temperature conditions. Such high pH fluids, however, exhibit longer shear recoveries, which may last several minutes.
There is therefore a need to provide compositions and methods that provide improvements in these areas.